Use of a rapid, sensitive immunoradiometric assay for thyrotropin to distinguish normal from hyperthyroid subjects

Using a new three-site immunoradiometric assay for thyrotropin (TSH), we measured concentration of this hormone in the serum of 47 patients with hyperthyroidism and 46 controls. The mean and range of serum TSH concentration was significantly lower in thyrotoxic than in control subjects, and it was possible to correctly identify 96% of thyrotoxic patients on the basis of a serum TSH concentration less than 0.5 milli-int. unit/L. We conclude that such measurement is highly sensitive for distinguishing hyperthyroid from normal subjects, and that the lower limit of normal for TSH in serum is about 0.5 milli-int. unit/L.